In recent years, environmental pollution has become a serious problem on a global scale, and exhaust gas from gasoline-driven automobiles is one source of air pollution. Thus, automobiles which emit reduced amounts of exhaust gas, and automobiles which emit no exhaust gas are being developed. As one automobile which emits reduced amounts of exhaust gas, there is a hybrid automobile that has combined an internal combustion engine with an electric motor. There are also automobiles which emit no exhaust gas, which use electricity. These automobiles are driven using, as an electric power source, electricity stored in a charge accumulating device such as a secondary cell or a capacitor.
Charge accumulating devices to be mounted in automobiles are required to be small and lightweight, and at the same time be capable of instantaneously charging and discharging massive currents. Thus, they are required to have a high output density. As one of the methods to obtain a high output density, there is a method of reducing resistance of various materials constituting the charge accumulating device (internal resistance of charge accumulating devices).
Generally, charge accumulating devices to be mounted on vehicles have a constitution in which, in order to enhance weight energy density, a positive electrode and negative electrode are formed in the form of a sheet, and the positive electrode and negative electrode are placed in a case in a rolled or laminated state via a separator formed similarly in sheet form. The sheet-formed electrode plate has the structure in which a mixture layer containing an active material has been formed on the surface of a metal foil which is intended to constitute an electric current collector.
In a charge accumulating device, aluminum foil is used in the electric current collector. Aluminum has an oxide coating, composed of aluminum oxide, formed on its surface. In other words, a common electric current collector, composed of aluminum, has an oxide coating. It has also been confirmed that when a charge accumulating device is driven and an electric current collector, composed of aluminum, is subjected to high voltage, a coating having high resistance is formed by reacting with a surrounding electrolytic solution.
Thus, the charge accumulating device had a problem that internal resistance increased due to an oxide coating or a high resistance coating which is formed on the surface of an electric current collector composed of aluminum. If the internal resistance increases, a voltage drop may occur when charging or discharging is conducted at a massive current, resulting in a decrease in the output of the charge accumulating device.
With regard to a passivation coating such as an oxide coating or a high resistance coating, Japanese Unexamined Patent Publication Nos. 7-22606, 2002-298853 and 2004-63156 have been disclosed.
Japanese Unexamined Patent Publication No. 7-22606 discloses that electronically conductive particles having a diameter smaller than the thickness of an aluminum foil are embedded on the surface of the aluminum foil. By embedding the electronically conductive particles in the aluminum foil on increase in internal resistance is suppressed.
Japanese Unexamined Patent Publication No. 2002-298853 discloses applying particulate carbons having a median diameter of 0.8 μm or less on the surface of the electric current collector. By attaching particulate carbons, the formation of a passivation coating at the interface between the electric current collector and an electrode active material or an electrolytic solution is suppressed.
Japanese Unexamined Patent Publication No. 2004-63156 discloses the formation of external surface of the electric current collector with hafnium or a hafnium alloy.
However, in the electric current collectors described in Japanese Unexamined Patent Publication Nos. 7-22606, 2002-298853 and 2004-63156, since the passivation coating on the surface of the aluminum foil has not been removed, i.e. the surface having an oxide coating formed thereon is subjected to further treatment, therefore, there was a problem that a sufficient effect of reducing internal resistance cannot be obtained.
Furthermore, in Japanese Unexamined Patent Publication Nos. 7-22606, 2002-298853 and 2004-63156, the aluminum foil and the conductive surface layer formed thereon have only been conjugated by the anchoring effect of ruggedness on the surface of the aluminum foil, and thus there was a problem in terms of durability and reliability.
On the other hand, Japanese Unexamined Patent Publication No. 2000-243383 discloses that the oxide coating is removed by polishing the surface of the aluminum foil in an oxygen-free atmosphere and then an active material layer is formed in the oxygen-free atmosphere. Even in the method disclosed in Japanese Unexamined Patent Publication) No. 2000-243383, the conjugation of the active material layer and the aluminum foil (electric current collector) is only by the anchoring effect, and had a problem that the binding strength is weak. Thus, there was a problem that the exposed region of aluminum of the manufactured electrode is oxidized, and thus enhancement in internal resistance is induced.
As an example of an aluminum foil having an electrically conductive material disposed on its surface, there is a product (trade name: Toyalcarbo, manufactured by TOYO ALUMINIUM K. K.) in which a carbon whisker was formed on an aluminum foil, using aluminum carbide as the seed crystal. However, aluminum carbide having high resistance exists between a carbon whisker and aluminum foil in this product, and the carbon whisker and aluminum foil are not directly bound, and thus the product had problems in terms of durability and reliability.
As described above, in aluminum foils used in the electric current collectors of conventional charge accumulating devices, there have been concerns regarding enhancement in internal resistance, and over durability and reliability because of the presence of an oxide coating on the surface.